[Stoves] Cajun Rocket (finned) Pot, and Eneron Finned Pot

Dean Still deankstill at gmail.com
Wed Nov 20 18:56:34 CST 2013


Dear Crispin and Dale,

To me, one of the most interesting parts of a discussion about pots is
starting to think of the pot as the heat exchanger. To get 45% thermal
efficiency with a wood stove (Tier 4) the pot has to be a very good heat
exchanger! Then a lot of the heat gets into the water reducing fuel used to
boil and simmer, reducing time to boil, and emissions. We see above 50%
thermal efficiency at high power by:

Optimizing heat flow through the bottom of the pot
Mostly convection but also radiation
Reducing mass in the pot
Using a 6mm channel gap around the pot with a 2.5 Kw fire
I wouldn't use more than a 25cm diameter pot (no lid) with such a small fire
The big bottom is good but in a straight sided pot the large exposed water
surface area can make it hard to achieve full boil

Baldwin's VITA stove got 53% thermal efficiency at hot start to boil
recently in Nepal. An amazing inexpensive stove/pot combination if you're
not trying for super clean.

Best,

Dean




On Wed, Nov 20, 2013 at 3:45 PM, Crispin Pemberton-Pigott <
crispinpigott at gmail.com> wrote:

> Dear Dale
>
>
>
> Much appreciated the test report for the other pot – that is a really big
> difference and more in line with the goal of your earlier and very
> interesting tests on finned pots.
>
>
>
> For those of you who have not seen Dale’s work look through the ETHOS
> website and you will find a couple of papers there.
>
>
>
> I can add a little to the discussion below. Dale points out that as the
> pots reach the boiling point, there is a change in the ratio of heat gain
> by the two different pots compared with the same ratio calculated for a
> lower temperature.
>
>
>
> Provided that the temperature in question is below, not at the boiling
> point (when other issues arise) this difference, noted as 23% instead of
> (10%, I guess) is entirely caused by not including the thermal mass of the
> pot. This difference in ratios does not arise if the pot is considered as
> well.
>
>
>
> Why? As the temperature nears boiling, and evaporation starts to really
> kick in, the % of total heat that is devoted to changing the temperature of
> the pot material starts to diminish. The temperature change rate decreases
> and most of the energy goes into evaporation. As I mentioned earlier, once
> the temperature stops changing the difference in heat transfer efficiency
> shows up in different evaporation rates (provided the heating power remains
> high so unmeasurable losses are relatively small).
>
>
>
> Interesting, yes? It shows clearly that the pot material and mass must be
> considered to be able to accurately determine the heat transfer efficiency
> (or any other efficiency that includes the pot) across any temperature
> range.
>
>
>
> The effect of asking ‘the same question’ under different circumstances can
> best be understood conceptually by creating a heat flow diagramme that
> shows where all the heat is going, and what portions of it are measured and
> compared. Dale has shown that asking the same question at low temperature
> and near the boiling point gives a significantly (measurably) different
> answer if the pot material is not considered.
>
>
>
> In the same way trying to get the ‘efficiency’ while simmering a hot pot
> at low power gives basically no useful result. What you *can* measure
> does not deliver useful information.
>
>
>
> Regards
>
> Crispin
>
>
>
> PS here are two slides on the matter: Low power hot pot v.s. high power
> cold pot.
>
>
>
>
>
> +++++++++
>
>
>
> All of Crispin’s comments are correct, and I suppose I should have
> included the mass of the pot and its energy gain to get the true energy
> transferred.  Still, my view is that whether the Cajun pot is 16% better or
> 10% better than a regular pot, that’s not enough to get excited about.  If
> someone want to redo the calculations I can send them the Word version of
> the report and they can edit it so that all the numbers are there.
>
>
>
> I’ve attached a report about another finned pot, the Eneron Pot.  This
> report covers 3 types of gas burners of the type used in restaurants.  I
> chose to test the Cajun pot rather than the Eneron because I believed the
> Cajun pot would be very good, and the Eneron not as good.  The first of
> these beliefs was wrong, so perhaps I should have tested the Eneron pot.
> Perhaps someone else would like to test it?  With a couple quick tests you
> could probably determine whether it is promising or not.  I might do that
> some day, as well as test variations of the Cajun pot.
>
>
>
> With regard to Ron’s questions, in my calculations I always included the
> evaporated water (except in the simulated open fire test where there was
> virtually no evaporation) but never the mass of the pot.  The formula for
> energy gained by the water would be:
>
>
>
> (Mass of water times temperature gain times 4.186) + (Mass lost through
> evaporation times 2250)
>
>
>
> Divide this number by the number of seconds elapsed during the test period
> gives you the power into the water in Watts.  This works for high or low
> power, or the overall test, or any other length of time.
>
>
>
> The estimated 700 Watts lost from the pot was not included in any
> calculations.  I gave that number as a way of explaining why the ratio of
> times to boil was greater than the ratio of heat transfer efficiencies.  As
> you approach the boiling point the net heat gain by the regular pot is
> about 1128 – 700 = 428.  The net heat gain by the finned pot is 1227 – 700
> = 527, or about 23% more.  If you had a pot that produced 701 W of heat
> transfer it would approach the boiling point quickly and then take a
> reeeeeeeeeeealy long time to fully reach boiling.  (It seems that half of
> my tests go this way!)
>
>
>
> Dale
>
>
>
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